Gas separation by pressure swing adsorption is achieved by coordinated pressure cycling and flow reversals over an adsorbent bed which preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure is elevated during intervals of flow in a first direction through the adsorbent bed, and is reduced during intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed component is concentrated in the first direction, while the more readily adsorbed component is concentrated in the reverse direction.
The conventional process for gas separation by pressure swing adsorption uses two or more adsorbent beds in parallel, with directional valving at each end of each adsorbent bed to connect the beds in alternating sequence to pressure sources and sinks, thus establishing the changes of working pressure and flow direction. This conventional pressure swing adsorption process also makes inefficient use of applied energy, because of irreversible expansion over the valves while switching the adsorbent beds between higher and lower pressures.
The prior art also includes the following pressure swing adsorption devices with cyclically operated volume displacement means reciprocating at the same frequency at both ends of an adsorbent bed, to generate pressure changes internally and thus improve energy efficiency.
Keller (U.S. Pat. No. 4,354,859) has disclosed a single bed pressure swing adsorption device for purifying both components of a binary gas mixture fed to a central point of the adsorbent bed. This device has volume displacement means which may be pistons or diaphragms, of specified unequal displacements at opposite ends of the bed.
My U.S. Pat. No. 4,702,903 discloses use of modified Stirling or Ericsson cycle machines for performing gas separations, in which expansion energy of the PSA cycle is recovered and heat may be applied directly through the modified Stirling cycle as a supplemental energy source to perform pressure swing adsorption gas separations.
My U.S. Pat. Nos. 4,801,308 and 4,968,329 disclose related gas separation devices with valve logic means to provide large exchanges of fresh feed gas for depleted feed gas. Such large feed exchanges, or effective scavenging, may be required when concentrating one component as a desired product without excessively concentrating or accumulating other components, as in concentrating oxygen from feed air containing water vapour whose excessive concentration and accumulation would deactivate the adsorbent.
My U.S. Pat. No. 5,082,473 discloses related multistage devices for with extraction and simultaneous concentration of trace components.
All of the above cited devices use reciprocating pistons or equivalent volume displacement mechanisms for establishing the cyclic pressure and reversing flow regime of PSA cycles. With relatively low PSA cycle frequencies attainable with conventional granular adsorbent beds, the reciprocating machinery is bulky and costly. Hence, there is a need for rigid high surface area adsorbent supports which can overcome the limitations of granular adsorbent and enable much higher cycle frequencies. High surface area rigid adsorbent supports, comprised of monoliths, stacked or spirally wound adsorbent-impregnated sheet material, are disclosed in my U.S. Pat. Nos. 4,702,903; 4,801,308; 4,968,329; and 5,082,473.
Small scale gas separation devices based on the above cited U.S. patents have been built and operated successfully, for applications including air separation and hydrogen purification. These devices all use mechanical pistons to generate the necessary reciprocating internal volume displacements, in a flow-regulated pressure swing adsorption cycle operating at relatively high frequency. Although adsorbent inventories are reduced compared to most conventional pressure swing adsorption systems, the piston swept volume must considerably exceed the volume of the adsorbent bed in order to generate the desired pressure ratio between minimum and maximum working pressures. In order to achieve the desired functions and energy efficiency, the piston drive mechanism must be adapted to exchange compression energy between adsorbent columns undergoing compression and expansion steps. With the cycle speeds permitted by commercial adsorbent pellets in packed beds (typically not exceeding a practicable limit of 50 RPM indicated by theoretical analysis and test experience), scale-up of such devices using pistons to larger scale tonnage air separation or hydrogen purification applications would be difficult owing to the large and heavily loaded low-speed reciprocating drive mechanisms which would be necessary.